Receding Ice, Rising Oceans

Astrobiology Magazine’s climate blog, The Hot Zone, recently discussed how receding glaciers could have catastrophic consequences for Earth’s climate. Increasing loss of glacial ice will lead to increasing water levels in the oceans. Ultimately, these changes could profoundly affect the biosphere. Be sure to add your own comments to the Hot Zone blog at: http://www.astrobio.net/blog/

Some of the most graphic examples of global warming are found in pictures of receding glaciers. Mount Kilimanjaro in Tanzania is one of the most dramatic. Compare photos from 1993 and 2000 to these from 2003 and 2004.

Researchers at Ohio State University report that Kilimanjaro’s glaciers could melt away within two decades.

When inland glaciers in tropical regions melt, they have an enormous effect on the environment. Species that rely on the habitat glaciers provide are threatened. Communities down the mountain, whose agriculture depends on a steady, predictable water flow from glaciers are likely to lose their livelihoods and food supply. Once the glaciers are gone, there no more water will flow into these communities. Glacial recession will lead to a shortage of fresh water.

Earlier this month, the The Met Office, UK’s National Weather Service, predicted that glacial retreat could lead to a 20 per cent decline in global agricultural productivity.

When it comes to orders of magnitude, though, Mt. Kilimanjaro is relatively small. Consider Greenland, with its 2.5 million gigaton ice sheet. The consequences of this vast amount of ice melting into the sea are mind boggling.

I can’t waste too much time being boggled, though. The enormity of glacial melt needs to start grokking – not just with me, but with everyone. Greenland, the largest island in the world, harbors about 10 percent of the world’s fresh water in its ice sheet. The Antarctic ice sheet, by way of comparison, holds 70 percent of Earth’s fresh water.

And now, it seems, the Greenland ice sheet is melting even more rapidly than it was ten years ago.

Using satellite data and sophisticated techniques that measure the smallest changes in the Earth’s gravity over the massive ice sheet, scientists from Utrecht University in the Netherlands, Bristol University in the UK, and the Jet Propulsion Laboratory in Pasadena, California, found that the Greenland ice sheet was losing mass three times faster now than it was during the early 1990s. During the 1990s, Greenland lost about 97 gigatons of ice mass per year. In 2007, it shed about 267 gigatons. Between 2000 and 2008, Greenland lost about 1500 gigatons of ice.

What is even more alarming, is the ice sheet losses could have been even higher. “Without increased snowfall and refreezing since 1996, the ice sheet mass losses would have been 100% higher,” said Dr. Jonathan Bamber, a professor at the University of Bristol, and one of the study’s authors.

I find numbers like this hard to picture. In cases like these, comparisons can be useful. I need a human scale or I can’t relate. For instance, I can relate to the size of a football field. I can walk the approximate circumference of an acre without having to be told where the boundaries are.

So, for the purpose of illustration, consider a single gigaton:

One gigaton is equal to one billion tons (or 10^15 g). It is equal in mass to about 2750 Empire State Buildings, or about 142 million African elephants.

One gigaton could provide enough water for 17 million people.

What is unnerving about these numbers they account for a substantial rise in global sea levels.

During the 1990s, Greenland’s ice sheet thaw accounted for about .46 millimeters per year of global sea level rise. Since 2006, ice loss at the rate of 273 gigatons per year caused a sea level rise of about .75 millimeters per year.

Over the nine years between 2000 and 2008, 1500 gigatons of melted Greenland ice produced a world-wide sea level rise of about 5 millimeters. If the entire Greenland ice sheet melted into the sea – which, at the present rate, could reasonably occur with in the next fifty years – the sea level would rise seven meters world wide.

About 1.2 billion people live in coastal areas around the world. Their homes, food supply, and infrastructure would be devastated. Think of New Orleans the day after Hurricane Katrina hit in 2005, but on a global scale.

To get a picture of what this will look like – and compare a seven meter sea level rise with, say, a ten or twenty sea level rise – have a look at this interactive Web site. You can play with the values yourself, and see what the world will look like if Greenland melts. Then imagine what the world would look like with no Antarctica – which has seven times more ice than Greenland.